[PATCH 5/5] drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

[Lyude Paul <lyude@redhat.com>]

Annoyingly, the VBT on the ThinkPad X1 Extreme 2nd Gen indicates that
the system uses plain PWM based backlight controls, when in reality the
only backlight controls that work are the standard VESA eDP DPCD
backlight controls.

Honestly, this makes me wonder how many other systems have these issues
or lie about this in their VBT. Not sure we have any good way of finding
out until panels like this become more common place in the laptop
market. For now, just add a DRM DP quirk to indicate that this panel is
telling the truth and is being a good LCD.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 drivers/gpu/drm/drm_dp_helper.c                       |  4 ++++
 drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 10 ++++++++--
 include/drm/drm_dp_helper.h                           |  8 ++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 2c7870aef469..ec7061e3a99b 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1155,6 +1155,10 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
 	{ OUI(0x00, 0x10, 0xfa), DEVICE_ID_ANY, false, BIT(DP_DPCD_QUIRK_NO_PSR) },
 	/* CH7511 seems to leave SINK_COUNT zeroed */
 	{ OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
+	/* Optional 4K AMOLED panel in the ThinkPad X1 Extreme 2nd Generation
+	 * only supports DPCD backlight controls, despite advertising otherwise
+	 */
+	{ OUI(0xba, 0x41, 0x59), DEVICE_ID_ANY, false, BIT(DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT) },
 };
 
 #undef OUI
diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
index 87b59db9ffe3..3d61260b08ad 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -325,11 +325,17 @@ intel_dp_aux_display_control_capable(struct intel_connector *connector)
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 {
 	struct intel_panel *panel = &intel_connector->panel;
-	struct drm_i915_private *dev_priv = to_i915(intel_connector->base.dev);
+	struct intel_dp *intel_dp =
+		enc_to_intel_dp(&intel_connector->encoder->base);
+	struct drm_i915_private *dev_priv =
+		to_i915(intel_connector->base.dev);
 
 	if (i915_modparams.enable_dpcd_backlight == 0 ||
 	    (i915_modparams.enable_dpcd_backlight == -1 &&
-	    dev_priv->vbt.backlight.type != INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE))
+	     dev_priv->vbt.backlight.type !=
+		     INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE &&
+	     !drm_dp_has_quirk(&intel_dp->desc,
+			       DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT)))
 		return -ENODEV;
 
 	if (!intel_dp_aux_display_control_capable(intel_connector))
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 51ecb5112ef8..a444209cd54b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1520,6 +1520,14 @@ enum drm_dp_quirk {
 	 * The driver should ignore SINK_COUNT during detection.
 	 */
 	DP_DPCD_QUIRK_NO_SINK_COUNT,
+	/**
+	 * @DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT:
+	 *
+	 * The device is telling the truth when it says that it uses DPCD
+	 * backlight controls, even if the system's firmware disagrees.
+	 * The driver should honor the DPCD backlight capabilities advertised.
+	 */
+	DP_DPCD_QUIRK_FORCE_DPCD_BACKLIGHT,
 };
 
 /**
-- 
2.21.0